In scientific research, a centrifuge is used to separate a mixture of substances, often in liquid form or suspension, into individual components according to their specific mass. This is accomplished by generating a high centrifugal force field which acts on the mixture causing heavier components to separate from lighter components.
A centrifugal force field is generated by spinning a sample mixture about a distal axis in a centrifuge. The centrifuge comprises a rotor, to which sample containers are attached for holding a sample mixture. The containers are generally pivotally attached to the rotor so they may swing outwardly under effect of centrifugal force maintaining the bottom of the container generally toward the direction of centrifugal force. This is necessary since the containers must contain liquid mixtures in the presence of gravitational force during inoperative periods of the centrifuge and in the presence of both gravitational and a centrifugal force field during operative periods. The pivotal mount permits the sample container to hang free, supporting a sample liquid in the presence of gravity, and permits the container to swing outwardly when the centrifugal force field is applied, to a generally horizontal position, to contain the fluid mixture against the combined forces where centrifugal force may be one hundred times the magnitude of gravitational force. The container is also generally removable from a centrifuge rotor for quick and easy replacement of samples and cleaning.
Thus, the mounting which connects the sample container to the rotor must provide pivotal freedom about an axis perpendicular to the centrifugal force field generated by spinning the rotor, and must permit removal of the container from the rotor without difficulty.
Generally, the greater the magnitude of centrifugal force field that can be generated, the more rapidly and accurately a centrifuge can operate to accomplish mixture separation. The magnitude of the centrifugal force field is limited by a number of factors. These factors generally relate to the speed with which the rotor of the centrifuge may be turned. Specifically, these factors may include the power available for driving the rotor, a diametral size of the rotor, the strength of the rotor construction, etc. For instance, increasing the radius of the rotor while maintaining the same rotational speed, increases the centrifugal force field. Similarly, for a rotor of a specific diametral size, increasing the rotational speed increases the centrifugal force field. Each of these variations, however, has characteristic limitations. Consider that the larger the diameter is of a centrifuge rotor, the greater the strength of the structure comprising a rotor must be to survive increases in centrifugal force at a selected rotational speed, due to increase in rotor radius and the effect of increased rotor structure weight.
Considering small bench top type centrifuges, much higher rotational speeds are required to generate sufficient centrifugal force fields to perform sample separation. Higher rotational speeds, however, generally result in much higher windage drag on the exposed surface of the outer portions of the rotor and mounted containers. Increased windage drag significantly increases power requirements for driving the rotor at the desired rotational speed. Since the diametral size of a rotor of a small tabletop centrifuge is determined by size constraints, as is the power source available for driving the rotor, the speed with which the rotor may be turned becomes dependent upon the windage drag generated. Windage drag effects are related to the streamline of the physical shape and structural characteristics of the rotor and container assembly. Because no, or only a partial, vacuum condition is usually present within most small tabletop centrifuges, the windage drag most often becomes a critical limitation to centrifuge performance.
Windage drag is determined by the surface area and the streamline of the shape of the rotor and container assembly. Thus, reducing the surface area or improving the streamline of assembly shape reduces windage drag and increases rotational speed under constant power. Through improvements in these factors, the force field generated may be increased.
In the past, due to strength considerations of rotor structure, the positions of the pivotal mounts for a sample container have been located radially outwardly between adjacent arms of the rotor, on a radial plane orthogonal with the axis of rotation and positioned through the portion of the rotor having maximal structural strength such that the arms of the rotor are maintained in radial tensile load when a centrifugal force field is generated and acts on the rotor mounting sample container and sample. Generally, this requires the containers to be pivotally mounted between the ends of a pair of adjacent rotor arms, outwardly from a yoke portion formed therebetween, to eliminate any problems of interference of pivotal movement of the container and mounting with the rotor.
The generally standard radial construction of a centrifuge rotor, however, exposes a great deal of rotor and container surface area at wide radial positions, which increases the effect of windage drag due to large surface exposure at high tangential velocity. With this construction, therefore, the rotor speed and thus the centrifugal force field which the centrifuge is capable of generating, is limited due to the size of the rotor structure which is necessary to provide sufficient strength to support the containers in the presence of the force field generated.